Previous work from this laboratory has shown that mRNAs coding for two muscle specific proteins, actin and myosin heavy chain, are present as cytoplasmic nonpolysomal ribonucleoprotein particles (mRNP) in embryonic muscle cells. The actin and myosin heavy chain mRNP particles were characterized with respect to its sedimentation profile in sucrose gradients, buoyant density, the relative RNA and protein contents and in vitro translation of the mRNAs isolated from these particles. About 30-40 percent of the total cellular mRNAs are present as cytoplasmic mRNP particles in embryonic muscle cells suggesting that these mRNP particles may be involved in translational control of myogenesis during the terminal differentiation of the muscle cells. At present we plan to study in detail the molecular basis of such control. We hope to extend these studies as probes in understanding the molecular mechanisms involved in the differentiation of muscle cells in culture.